The present invention relates to a magnetic disk apparatus for carrying out recording and reading operations for a magnetic disk having information recorded on part of its data surface for aligning the magnetic head with the center of a track and, more particularly, to a magnetic disk apparatus having a circuit for detecting the recorded position of the servo information on the target track of the magnetic disk.
A magnetic disk apparatus of this kind, as described in the pending U.S. patent application Ser. No. 07/219,545 filed on July 15, 1988 by the same applicant, is used for performing a recording and reading operation on a magnetic disk having on its data surface a plurality of concentric circular tracks and, at the leading edge of each sector, a servo area in which servo information for aligning the magnetic head with the center of the target track in the track following mode. To distinguish the servo information from the data on the target track, a servo ID (identification) is recorded at the leading edge of the servo area. Upon detection of this servo ID after aligning the magnetic head by open loop control in the track seek mode, the magnetic disk apparatus reads the succeeding servo information recorded across a track boundary. The apparatus then detects the amount of positional deviation of the magnetic head from the center of the target track according to this servo information, and adjusts the position of the magnetic head correspondingly to that amount in the track following mode.
Servo ID's are recorded at a predetermined recording frequency. The magnetic disk apparatus, in order to detect these servo ID's, has a servo ID detecting circuit including a band-pass filter (separating circuit) passing the component of the servo ID recording frequency. The generation of a sampling signal for reading servo information is triggered by the detection of the amplitudes of signals having passed this filter to be greater than a predetermined amplitude.
When information recorded in the data area of a magnetic disk is read by a magnetic head, the amplitude of the reading signal will be greater if the intervals of magnetization inversion are shorter, whereas the amplitude will be smaller if the intervals are longer. As a result, a reading signal having the same frequency as that of the servo ID, or a frequency close to it, may be generated depending on the pattern of data recorded in the data area. The conventional magnetic disk apparatus described above is so structured that the component of the frequency recorded in the servo ID area is separated by the band-pass filter and signals whose amplitude reach or surpass a certain level are recognized as the servo ID. This results in the disadvantage that data in the data area may be mistaken for the servo ID, so that the magnetic head positioning motor is controlled with a reproduced data signal mistaken for servo information.